Representative of a method for A/D conversion, for the transforming of an analog input voltage into a digital number proportional to the amplitude, is the count method. Embodiments of the count method are the single-ramp method and the double-ramp method, wherein, by means of an integrating component, in the case of the first-mentioned method, only the input voltage is integrated, while, in the case of the second-mentioned method, both the input voltage and the reference voltage are integrated.
The period length for the A/D conversion of a measurement signal is determined by the sum of the discharging time t1 and the charging time t2 of a capacitor, which is arranged in the feedback loop of the integrating component. The switching between the charging phase and the discharging phase occurs, in each case, at that point in time, when the output voltage of the integrating component reaches a predetermined threshold voltage. This is implemented via a downstream comparator, whose one input receives the output voltage of the integrating component and whose other input receives the threshold voltage.
For measuring the period length of the pulse-width modulated output signal, the time between two rising, or two falling, edges of the pulse-width modulated, output signal is registered by means of a counter. The resolution in the determining of the period length is, in such case, dependent on the frequency of the counter clock signal—the higher the counter clock frequency, the better is the time resolution. If, for example, the counter is operated with the counter clock frequency f, then the following holds for the smallest measurable time change Δt:Δt=1/f. 
Known A/D converters become naturally more expensive, as their resolution becomes higher. Moreover, the resolution of known A/D converters is usually fixedly predetermined, so that, usually, an A/D converter optimally adapted for the particular case of application is installed.
In process measurements technology, the tendency is in the direction of multiparameter measuring devices—thus measuring devices able to measure a plurality of different, physical or chemical, process variables. Examples of such process variables include e.g. temperature, pressure, fill level, flow, pH-value, ion concentration, conductivity, etc. of a medium to be measured. The quality of a measurement is determined, in principle, by the measurement accuracy and/or the time resolution of the measuring, thus the measurement rate. Usually, these two requirements work against one another. Thus, for a highly accurate measurement, a correspondingly long time is needed. In contrast, if the time for a measurement is limited, then the increased rate of measurement is at the cost of measurement accuracy. The escape route is represented by A/D converters, which are appropriately fast and which, at the same time, have a high bit-resolution. The disadvantage of these converters is that they are relative expensive, which, in turn, drives the manufacturing costs for the measuring devices significantly upwards.
Exactly in the case of multiparameter measuring devices, yet another aspect enters the picture: In process measurements technology, different requirements, as regards measurement accuracy and as regards time resolution, are placed on the providing of the measured value, i.e. on the A/D conversion of the measurement signals representing the individual process variables. In terms of an example, consider temperature, a process variable, which usually changes relatively slowly as a function of time, since, in the normal case, no abrupt temperature jumps arise. However, exactly in the case of temperature measurements, usually a high measurement accuracy is required. Mentioned in this connection is the monitoring and/or control of temperature in a chemical process. Since the temperature changes relatively slowly, relatively much time is available for registering the measured value.
Quite different is the situation e.g. in the case of a pressure measurement. In this case, it is necessary also to be able to detect suddenly occurring, pressure jumps. This is only possible, when the measurement duration is correspondingly short; in the case of many pressure measurements, however, the accuracy of measurement is of lower importance. It makes sense in this connection then to find a compromise, or a flexible solution, as the case may be, which permits an adapting of the registering of the measured value, as regards both measurement accuracy and measurement duration, to the particular requirements of a given application.